The invention relates to a method for determining constituents in water and to an apparatus for performing the method.
To determine the content of certain constituents of water--and hence the quality of water and particularly of wastewater polluted with organic substances and/or nitrogen compounds and/or halogen compounds--it is known to evaporate a sample in an atmosphere of an inert transport gas enriched with oxygen, burn it, and deliver the resultant combustion gas mixture to a detector suitable for detecting carbon dioxide, nitrogen oxides, and so forth.
Proven detectors include (among others) infrared detectors for the carbon content, special chemoluminescence detectors for the nitrogen oxide content and so-called coulometric detectors for the halide content.
Detection methods based on combusting a water sample for detecting the content of organic constituents--the so-called TOC (total organic carbon)--have gained wide use. Typically, a small amount of water with the transport gas is delivered to an oven that is heated to a predetermined temperature by a resistance heater; in the oven, it evaporates and burns virtually all at once, and the combustion gas is delivered to an NDIR-CO.sub.2 detector, whose CO.sub.2 content indication is a measure of the carbon content of the water sample. An advanced version of this method and corresponding apparatus are described in German Patent DE 43 44 441 C2. A modified arrangement for measuring very low TOC values--for instance in high-purity water or high-purity solutions for medical applications--is described in European Patent Disclosure EP 0 684 471 A2.
By this method, the TOC of interest is not readily determined; instead, it is in principle the total carbon content of the water (TC=total carbon) that is determined, which along with the TOC includes the component of inorganic carbon compounds (TIC=total inorganic carbon). Yet to determine the TOC, these carbon compounds are removed in a preceding stripping step; see for instance German Patent DE 39 42 229 C2 (which includes further references to the literature).
When the inorganic carbon is stripped off by expulsion, the further problem arises that expellable or volatile organic carbon (POC or VOC=volatile organic carbon) is likewise removed from the sample. In German Patent Disclosure 43 09 646 A1, a method and a testing apparatus of the above-outlined type are therefore proposed in which the POC content is measured separately and, to obtain correct POC measurement values, is caught inadvertently along with expelled carbon compounds by a special adsorber reagent.
In practical execution, on the other hand, for samples for containing solids, the total content of organic constituents cannot be determined, which additionally makes the measurement results incorrect. To avoid excessive sudden pressure loads on the analysis apparatus from deflagration in the hot oven, the water sample quantities delivered must in fact be made quite small, which necessitates the use of superprecision metering technology. To protect them, on the other hand, precision filtration of the sample must be performed, which excludes a substantial proportion of the solids, and thus the total quantity of organic constituents, from the content determination. The already only moderate detection precision, because of the small sample quantities, thus becomes unacceptably poorer for many applications, such as wastewater analysis.
In this last problem, one method provides some remedy, in which an aqueous sample is heated relatively slowly in an initially cold heating vessel and combusted in discontinuous analysis operations; see German Patent DE 44 12 778 C1. The CO.sub.2 content of the combustion gas, in this discontinuous method, must be followed and integrated over a certain period of time. This method avoids major pressure fluctuations in the apparatus, and therefore larger sample quantities can be used. However, the need for following the time dependency of the CO.sub.2 concentration down into the range of very low values requires a highly sensitive and correspondingly expensive detector that still functions precisely even in that range, and a precision integrator. The equipment expense is increased still further by the provision of a second heater.